A circuit arrangement for a free voice telephone by means of which uncoupling between the two transmission paths is achieved is known from DE-OS No. 36 06 973. By means of a voice control circuit, which favors transmission in one transmission path and hinders it in the other transmission path as a function of the voice level in both transmission paths, the possibility is provided for controlling the band width of the two transmission paths by means of controllable filters. In this manner a greater portion of the frequency band is put at the disposal of the transmission path with the higher voice level than to the transmission path with the lower voice level. This is done by changing the impulse frequency of a voltage-controlled oscillator as a function of the voice level values. Limit frequencies occurring during silence intervals are shifted when the signal level rises in such a manner that the receiving (i.e., incoming) direction can transmit only at the lowest frequencies when the signal level in the sending (i.e. outgoing) direction is higher, and the sending direction can only transmit at the highest frequencies when the signal level in the receiving direction is higher. It is not entirely impossible in this process that the frequency shifts impair voice quality, at least in the transition phases.
DE-OS No. 35 28 973 discloses a circuit arrangement for voice controlled attenuation of two signal paths with opposing directions of transmission with incorporated attenuation devices. The signal of one signal path is in each case taken off a controllable amplifier and is used to control the attenuation in the other signal path. This makes it possible to further attenuate the transmission path with the weaker signal and to attenuate the signal path with the stronger signal to a lesser degree. Since such regulating processes must constantly change and readjust in the course of a connection in two directions, especially in telephone connections, it may occur with such reversing events that a portion of the voice signal is lost. To avoid this, additional delay circuits are incorporated with the circuit arrangement described in DE-OS No. 35 28 973. Such delay circuits have, however, a non-negligible effect upon the overall propagation time, so that difficulties may occur in overseas connections for instance, when the type of connection alone already produces considerable signal propagation times.
The German periodical "Aarchiv fur Elektronik und Uebertragungstechnik" volume 39, (1985) No. 2 contains an article starting on page 123 with the title: "Concerning the Development of Filters for ideal QMF and Polyphase Filter Banks". This is a process by means of which an audio frequency band can be divided into individual frequency sub-bands by means of an analysis filter bank. It is also possible to reconstruct the original signal from the individual frequency sub-bands by means of a synthesis filter bank working on similar principles. Analysis polyphase filter banks can also be used as demultiplexers for signal channels contained in a frequency multiplexed signal in order to select the individual connections offered in different frequency sub-bands. Furthermore it is possible to use polyphase filter banks when the problem comprises using transmultiplexers in order to convert frequency multiplexed signals into time multiplexed signals or to convert time multiplexed signals into frequency multiplexed signals.